When digital images are resampled, screened and finally printed on a printing device utilizing halftones, jagged edges and subject-moiré are sometimes introduced in the printed image. The introduction of jaggies and subject-moiré occurs when the image contains frequency patterns which are near to, or higher than, the frequency of one of the halftone screens of the printing device. This phenomenon is generally known as aliasing.
Antialiasing in the context of digitizing line art and certain graphical image structures is a method of using intermediate levels of intensity to achieve subpixel position of edges for several reasons including reduction or elimination of jaggies on the edges of lines and polygons, including text. Antialiased refers to those segments or regions of an image that are effected by an antialiasing operation applied to the image (e.g., an image processing operation or a physical process resulting in gray pixels along the edges of line art or text). Jaggies are primarily visible at the edges of sloped lines approaching horizontal or vertical.
A number of patents and publications have disclosed information relevant to antialiasing. For example, “A Comparison of Antialiasing Techniques,” IEEE CG&A, Vol. 1, No. 1, Jan. 1981, pp. 40-48, F. Crow suggests that prefiltering is a computationally effective technique for antialiasing. U.S. Pat. No. 5,432,898 to Curb et al, issued Jul. 11, 1995, describes a system and method for anti-aliasing of lines within a data processing system having graphics capability. U.S. Pat. No. 5,438,656 to Valdes, et al., issued Aug. 1, 1995, describes a method of synthesizing multi-level raster shapes directly from ideal shapes. U.S. Pat. No. 6,144,461 to Crean, et al., issued Nov. 7, 2000, describes a method for antialiased tagging (AAT) applied coincident with an antialiasing operation, in which a different filter is used for each image content type (e.g., text, graphics).
A common method for reducing aliasing is to apply a low pass filter (an anti-aliasing filter) to the original image before it is being resampled, or another low-pass filter prior to halftoning. The problem with applying either low-pass filter is that low-pass filters reduce effective resolution. Jaggies as well as subject-moire patterns are generally most severe when the image content is aligned with one of the two axial directions of a halftone screen. This is particularly true in the case of line screens, where there is only one axial direction. Low-pass filtering of an image prior to halftoning lowers the effective resolution and quality of an image isotropically, when it may not have been necessary since the resulting interference patterns are highly directional. It would be desirable to have an optimal filter such that the jagged edges and visual artifacts disappear, while blurring the image as little as possible.